It is generally accepted that a good semi-conductor should have a certain number of essential characteristics if it is to function well, the most obvious characteristics being:
(a) a large variation in resistance under the action of the agents considered separately or otherwise; PA1 (b) the lowest possible detection threshold; PA1 (c) a resistance that is in a range compatible with a reliable electronic circuitry and if possible with one that is not expensive, for the higher the resistance, for example greater than 10.sup.10 .OMEGA., the more costly will be the necessary electronic circuitry and the more the stray current phenomena (induction) will interfere with the measurements; on the other hand, if this resistance is too low, for example lower than 10.sup.6 .OMEGA., the response to the agents will become extremely low and the detection threshold will be too high.
Various detection systems are already known, and they may be arranged in three broad categories based on the following principles:
the detection of fumes and smoke from combustion results from a variation in the current produced in a photocell which reacts during the passage of the smoke through its light path. Various more or less sophisticated systems have been produced on this basic principle, in which for example at least two photocells connected in a bridge circuit, chemical products added to make the smoke less transparent, or one or more smoke passage chambers have been used . . . ;
the use of the Tyndall effect of lateral diffusion of the detection light to show changes in radiation;
the smoke, fumes and gases from a combustion are detected by means of ionized preparation or radioactive substances.